This invention relates to a temperature measuring apparatus for detecting temperature of an object and, more particularly, a temperature measuring apparatus suitable for a microwave oven.
In a microwave oven, magnetron energizing control for generating microwave energy to heat food is carried out in response to food temperature. A conventional temperature measuring apparatus for a microwave oven comprises an infrared sensor, a chopper, a chopper temperature sensor, and a signal processing circuit. More particularly, the food temperature is usually detected indirectly with the infrared sensor, which consists of an infrared-sensitive material LiTaO.sub.3, for instance, and is provided on the ceiling of a heating chamber with its detecting surface directed toward the food. Infrared radiation emitted from the food is intermittently blocked by the chopper which is provided between the infrared sensor and food. While the radiation from the food is blocked, it does not reach the infrared sensor. During this time, however, a certain amount of infrared radiation is emitted from the chopper. The infrared sensor thus detects infrared radiation from the food and that from the chopper alternately, so that it produces an AC output signal varying between alternate high and low levels according to the amount of the incident infrared radiation. The chopper temperature sensor comprising a thermistor, for instance, is provided separately to obtain a signal representing the chopper temperature or ambient temperature of the chopper. The signal processing circuit carries out arithmetic operations (subtraction/addition) in an analogue manner so as to derive a food temperature signal for the magnetron energizing control from output signals of the infrared sensor and the chopper temperature sensor. However, the infrared sensor output y does not vary with a linear function y=x.sub.f of the food temperature x.sub.f (.degree.C.) as shown by a dashed line in FIG. 1 but varies with a function y=f(x.sub.f) shown by a solid curve.
The infrared sensor output voltage is also subject to the chopper temperature. If the chopper temperature x.sub.cp (.degree.C.) is taken into account and the temperature measuring apparatus operates on condition that the infrared emissivity of the chopper is considered to be substantially identical to that of the food while the chopping duty cycle is 50 percent, then the infrared sensor output y is given as EQU y=.vertline.f(x.sub.f)-f(x.sub.cp).vertline. (1)
where the absolute value expression in equation (1) represents an output of a full wave rectifier in the signal processing circuit. A dot-dash curve shown in FIG. 1 indicates the equation (1). The solid curve for y=f(x.sub.f) is obtained when f(x.sub.cp)=0, i.e., when the chopper temperature is 0.degree. C.
The infrared sensor output y given by the equation (1) greatly differs from the linearized plot for y=x.sub.f. In addition, the sensitivity characteristics of the infrared sensor are basically different from those of the chopper temperature sensor. Accordingly, the conventional temperature measuring apparatus above-mentioned can not always provide an accurate food temperature signal over a wide temperature range.